Frequency modulated carrier wave receiver circuit



Dec. 21, 1943. s U -r 2,337,392

I FREQUENCY MODULATED CARRIER WAVE RECEIVER CIRCUIT Filed June 19, 1941 INVENTOR diynwarfi m w-M A-r-r'oRNEY Patented Dec. 21, 1943 FREQUENCY IVIODULATED CARRIE-R WAVE RECEIVER CIRCUIT Seymour Hunt, Flushing, N. Y., assignor to Radio Corporation of America, a corporation of Dela- Application June 19, 1941, Serial No. 393,704

13 Claims.

My present invention relates to frequency modulated. carrier wave receivers, and more particularly to novel methods of, and means for, receiving angular velocity-modulated carrier waves.

In the past various devices have been described for detecting angular velocity-modulated carrier waves such as frequency, or phase, modulated Waves. In general, these devices have utilized discriminator networks for translating the wave of constant amplitude and variable frequency, or phase, into a wave of variable amplitude, a rectifier then being employed to derive the modulation voltage from the variable amplitude carrier wave. It may be stated that it is one of the main objects of my present invention to provide circuits which function in a manner different from those of the prior art, and these circuits may generally be described as embodying an oscillator circuit normally producing oscillation waves of constant amplitude and constant frequency, the

normal oscillation frequency being chosen substantially equal to the center, or mean, frequency of the applied waves of constant amplitude and variable frequency whereby the locally produced oscillation waves are modulated in frequency in the same manner as the applied waves.

Another important object of the invention may be stated to reside in the provision of a detector circuit for frequency modulated (FM) waves wherein a single electron discharge tube has certain of its electrodes arranged to function as an oscillator operating at substantially the center frequency of the applied FM waves, there being derived from the output circuit of the tube the modulation voltage corresponding to the modulation originally applied to the carrier at the FM transmitter. I

Another object of the invention may be stated to reside in the provision of an FM discriminator which comprises an electron discharge tube having certain electrodes functioning as an oscillator operating at substantially the center frequency of the applied FM waves, and there being developed in the output circuit of the tube modulated carrier waves whos carrier frequency is equal to the center frequency and whose amplitude is variable in accordance with the modulation originally applied to the carrier at the FM transmitter.

Still other objects of my invention are to provide detector circuits for angular velocity-modulated carrier waves which have this generic feature in common: an oscillator of constant amplitude and fixed frequency is caused to produce oscillations corresponding to applied angular velocityemodulated carrier waves, and the oscil lations so produced are caused to control an electron stream to an output electrode in accordance with the vector sum of the modulaed wave voltage and oscillator voltage whereby in the output circuit there may be derived variable amplitude effects corresponding to the modulation originally applied at the carrier transmitter.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be under stood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a circuit embodying one form of the invention,

Fig, la shows a modification of the arrangement in Fig. 1,

Fig. 2 shows another embodiment of the invention,

Fig. 3: shows the discrimination characteristic of the circuit.

Referring now to the accompanying drawing, wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1 the circuit of a receiver arrangement which was actually constructed and operated to receive signals in the FM band of 42 to 50 megacycles mc.) The PM signals, which may be collected for example with the usual dipole I, were applied to the input coil 2 of an electron discharge tube 3 of the GAS type, Of course, any other type of tube may be used, such as 6K7 or .6S A7. The FM signals were applied to coil '2 through a primary winding 2. The 6A8 type of tube, as is ,well' known to those skilled in the art, comprises a cathode 4, an output electrode, or plate, 5 and five intermediate cold electrodes arranged in successive order in the electron stream flowing from cathode 4 to plate 5. The firstof these grids 6 is the input grid, and was connected to the high potential side of input coil 2 through a direct current-blocking condenser I. The cathode 4 was connected by lead i to an intermediate tap on input coil 2, as at the midpoint. The low-potential side of input coil 2 was grid 6 voltages derived from denser 1 may have a value of 100 to 300 micro.-

microfarads (mmf.). The second control electrode H were actually a pair of spaced rods,"

and was connected to a point at +250'volt'sthrough a resistor E2 having a. magnitude of;

about 20,000 ohms. a g The electrode H functions as the anode elective to the center or mid-band frequency Fe.

FM waves, yet they differ in phase due to the frequency variation of the applied FM signals. Due to the phase difference between the voltages applied to grid 6 there exists effectively on the grid the vector sum of the two phase-differing voltages. It is this resultant voltage which controls; the electron stream flowing to the plate 5. In Fig. 3 there is shown the variation in amplitude of the plate current as the input wave carrier frequency is deviated back and forth relaother words Fig. 3 shows the discrimination characteristic of the oscillator circuit. It is pointed trode of a local oscillator circuit which includes 2 the cathode 4, the input grid 6 and'the oscillator 1 anode ll. Regenerative feedback for oscillation 1 production is provided by connecting the cathode '4 to the tap on the input coil 2. This provides a Hartley oscillator circuit well known to those skilled in the art. The upper end of resistor I2 is by-passed for the oscillation frequency current by the condenser l2. Thethird and fifth grids were connected together to provide a. screen electrode, designated by the numeral I3, and were connected to a positive voltage source of +100 volts, the screen being by-passed to ground for the ultra-high frequency currents by a condenser.

The fourth grid l5, located between the positive screen grids 13, was connected to ground.

The plate 5 was connected to a source of positive voltage of approximately +250 volts through the primary winding of an audio output trans- "former iii, the voltage supply lead being bypassed to ground by an ultra-high frequency bypass condenser l6. The secondary winding of audio transformer 16 was connected to an audio voltage utilization means; It'was found, for example, that undistorted and loud signals could be heard by merely connecting head phones across the output circuit of tube 3. 'Howeven'the output of audio transformer [6 when plugged into the audio input terminals of the audio amplifier of a normal broadcast receiver, such as the audio amplifier used to amplify the output of a phonograph pick-up of a combined radio-phonograph receiver, gave an undistorted and satisfactory amplification sufficient for loudspeaker reproduction. In other Words, the tube 3 not only functions to detect the applied FM signals,

but produces in the output-circuit of the tube a highly amplified modulation voltage. Due to the linear discrimination characteristic, as shown in "Fig. 3, the output of tube 3 is a faithful reproduction of the modulation.

Inoperation, the'variable condenser 9 need only be adjusted to substantially the center, or the carrier, frequency of the desired particular FM channel. The oscillator section oftube 3 functions normally, and in the absence of applied FM signals, to produce oscillations of' constant amplitude and a frequency substantially equal'to the carrier frequency of the FM signals to be detected. Th theoretical aspectsof the operation involved in, this detection circuit will now be given. I do not wish to be limited to the theoretical explanations offered in this description to explain the operation of the circuit. l

It would appear that there is applied'tothe the applied FM waves and the local oscillator. Although these voltages have substantially the same frequency at the carrier, or center, frequency of the applied work I-lll. is so chosen as to quench the oscillations of the local oscillator section at a super-audible rate.

tron stream.

out that between the spaced peaks of the curve there exists a perfectly linear portion which makes it possible to get faithful reproduction of the modulation signals.

The detection of the variable amplitude-constant frequency current is accomplished by the super-regenerative detection produced by net- The time constant of network 'l-l 0 The time constant should be so chosen that maximum regeneration occurs in the oscillator section without giving any audible indication of its presence. Since the oscillator is operating in the ultra-high frequency spectrum, the quenching rate can be above audibility without in any way making it diificult to give to the network ll0 a sufficiently long time constant to secure maximum gain in the output circuit. It is found that the time constant of '|l0 can be made long enough so as normally to render th quenching frequency audible (say of the order of 10,000 cycles) thereby to further increase the gain, and yet by thereduction of'the value of the screen condenser the audible quenching frequency can be" degenerated to inaudibility. It will, therefore, be appreciated that the super-regenerative "detection action in the circuit of Fig. 1 not only functions to provide the modulation voltage in the output circuit without the use of any specific 'rectifien'but, also, provides a high degree of gain so that the output circuit can drive an audio network of as little asone audio amplifier and. a reproducer. In summary, then, the oscillator section of tube 3 functions as a locked-in oscillator driven by the applied FM signals and theplate circuit of tube 3. For example, an output resistor, by-passed for carrier currents, may be used to couple the plate of tube 3 into an audio stage. Furthermore, a push-pull audio 'sta ge may be driven from separate resistors arranged respectively in the plat and screen circuits,'each output resistor in that case being bypassed for carrier currents. This can be done because the modulation voltage is developed in any of the cold electrodes disposed in the elec- The circuit 29 will have a pass band sufficient to transmit the entire FM channel swing, and the latter at present is up to 150 kc. actual-tests it has been found that the linear portion of the characteristic of Fig. 3 extends deviations of the applied FM signals.

ages-7,392 over a band of 500 kct, at 42' me. It is to be understood that a-tuned ultra-high frequency amplifier stage may" be inserted ahead of the tuned circuit 2& so as'to' minimize any oscillator radiation which may b found to exist. Such an amplifier-would also provide an increased gain at the grid '6; It is, of course, not necessary that there be applied to input coil 2 the FM signals radiated from the transmitter. For example, the coil 2- and condenser 9 could be fixed to resonate at an intermediate frequency (1. F.) a

in a superheterodyne type of receiver. In that case the'received FM signals would be put through the usual conversion step to produce the I. F.

energy, and then the FM'signals whose center frequency is at the I. F. value would be applied 9 and 9' denotes the mechanism which tunes 29 and 92 to the carrier frequency of the applied FM Waves. Otherwise the circuit is'exactly the same as in Fig. 1-. The insertion of circuit 9-2" results in a higher order of gain at audio over the arrangement of Fig. 1'. The signals-may be applied to either the-grid circuit or the cathode circuit.

In Fig. 2 there is shown a circuit which may be employed where it is desired merely to have the variable amplitud currents produced in the plate circuit of tube 3'. In other words, in such cases where it is desired to have discrimination followed by separate rectification, it is merely necessary to adjust network 1-49 so that its time constant is sufficiently short to prevent super-regeneration. The oscillations will not be quenched in this case. In such case detection will not take place as described above, but there will flow in the plate circuit of tube 3 high frequency currents of variable amplitude produced in accordance with the characteristics shown in Fig. 3.

In Fig. 2, then, it may be possible to transform an FM wave into an amplitude modulated carrier wave which may subsequently be rectified in the usual manner to produce the audio frequency voltage. To accomplish this, itis merely necessary to change the circuit shown in Fig.

1 in the manner of Fig. 2. The leak resistor i and condenser l in that case are given a very low time constant value, while there is arranged in circuit with plate 5' a resonant circuit 30 which is tuned to the same frequency to which the input circuit 2-9 is resonated. In other words. if the condenser 9 is variable and the radiated FM signals are directly applied to the input coil, then the condenser of circuit 30 will be variable concurrently with condenser 9' so that circuit-30 and circuit 2-9 will be adjusted to the same carrier frequencies of th FM band. Preferably, there should be applied FM signal at intermediate frequency to coil 2, and'in that case circuit 30 will be fixedly tuned to the operating 1. F. value.

It is found that in this case the oscillator section of tube 3 functions in the manner of a "locked-in oscillator whose oscillations are caused to vary in frequency with the freguency s explained previously the platecurrent', in the absence of the super regenerative detection action, varies as shown in Fig. 3. There is, therefore, produced acrosscircuit 30 amplitude modulated carrier Wave voltage which is impressed upon the input circuit 3-! of afollowingdiode detector circuit 40. The circuit 3| is, of course, tuned to the same frequency as circuit 3i], andv the modulation, or audio, voltage is developed. across load resistor 59 which is by-passed by the usual carrier by-pass condenser 5 I I While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be mad without departing from the scope of my invention, as

set forth in the appended claims.

What I claim is: r 1. In combination with an electron discharge tube including a cathode, an output electrode and a plurality of intermediate control electrodes arranged in successive order in' the electron stream between th cathode and output electrode,

an oscillatory network connected between the cathode and two of the intermediate electrodes to provide an oscillator resonant to substantially the center frequency of applied angular velocitymodulated carrier waves, means for applying. such waves to said oscillator network, a condenserresistor circuit in said oscillatory network having a time constant sufficiently long to provide super-audible quenching 0t oscillations thereby to produce super-regenerative detection. actionin said oscillator, a circuit for providing the modulating signal Voltage of the said applied modulated waves, and means for connecting said modulating signal voltage circuit to said output electrode.

2. In combination with an electron. discharge tube including a cathode, an output electrode and a plurality of intermediate control electrodes arranged. in successive order in the electron bility, and a modulation signal Voltage output circuit connected to said output electrode.

3. In combination with an electron discharge tube including a cathode, an output electrode and a plurality of intermediate control electrodes arranged in successive order in the electron stream between the cathode and output electrode, an oscillatory network connected between 'the cathode and two of the intermediate electrodes toprovide an oscillator normally producing oscillations of constant amplitude and of a frequency equal to the center frequency of applied angular velocity-modulated carrier waves, means for applying such waves to said oscillator network, means for rendering said oscillator operative as a super-regenerative detector, and a modulating signal voltage output circuit connected to said'output electrode.

4. In combination with an electron discharge tube including a cathode, an output electrode and a-plurality'of intermediate control electrodes trode. 8. In combination arranged'in successive order in the electron stream *between the cathode and output electrode, an oscillatory network connected between the cathode and two of theintermediateelectrodes to provide an oscillator normally producingoscillations of constant amplitude andof a frequency equal to substantially the Center frequencyof applied angular velocity-modulated carrier, waves, means .connected solely in said oscillatory network to render the oscillator operative as a super-regenerative detector, means for applying such Waves to said oscillator network, a. modulating signal voltage output circuit contioning as an oscillator section and provided with p a second electronic section functioning as an output voltage section, anoscillator circuit, tuned to substantially the center frequency of applied frequency modulated carrier waves, connected to .said oscillator section electrodes, mean forepplying frequency modulated carrier waves to said oscillator circuit, a resistor-condenser circuit having a long time constant connected in said oscillator circuit thereby to provide super-audible quenching of oscillations, and a modulation signal voltage output circuit connected to the output electrode of said output section.

6. A method of detecting angular velocitymodulated carrier waves which includes locally and. continuously producing in theabsence of such waves oscillations ,of fixed amplitude and of fixed frequency substantially equal to the center frequency of said modulated waves, collecting angular velocity-modulated carrier waves,

modulating the. frequency of said locally produced oscillations in accordance with the ,frequency of the said collected waves,,providing, an electron stream, and intermittently modulating said electron stream with said frequency modulated-locally produced oscillations thereby to provide an electron current whose intensity varies in accordance with th modulation signal of, said collected waves.

7. In combination with an electron discharge tube including a cathode, an output electrode and a plurality of intermediate control electrodes arranged in successive order in the electron stream between the cathode and 0utput' .e1ectrode, an oscillatory network connectedlbetween th cathode and two of the intermediat electrodes to provide an oscillator. normally producing oscillations of constant amplitude and of a frequency equal to the center frequency of applied angular velocity-modulated carrier waves,

means for applying such waves to said oscil-,

lator network, a'grid leak condenser circuit for providing super-regenerative detection in'said oscillator network, and a modulating signal voltage output circuit connected to said output elecwith an electron discharge tube, including a cathode, an output electrode and a plurality of intermediate control electrodes,

arranged insuccessive order in theelectron stream between the cathode and output elec- -trode, a network connected between the catliode and two of the intermediate electrode to provide a local oscillator normally producing osamnes a cillations of ;cons tant amplitude and .of a frequency equal to the center frequency of applied angular velocity-modulated carrier waves, means .for applyinglsuch waves to said oscillator, network, a super-audible quench circuit connected to said oscillator network,-and a modulating signal voltage output circuit connected to aid'output electrode. l

9. In combination wa ts electron discharge tube provided with a plurality of electrodes runetioning as an oscillator section and provided with a second electronic section functioning as an output voltage section, a local oscillator circuit, tuned to the center frequency of applied frequency modulated carrier waves, connected to said oscillator section electrodes, said oscillator circuit including a super-audible oscillationquenching means and producing oscillations in the absence of said applied waves whichfare of constant amplitude and of said center frequency, means for applying frequency modulated carrierwaves to said oscillator circuit, and a modulation signal voltage output circuitconnected to the output electrode of said output section.

10. A method of detecting angular velocitymodulated carrier waves which includes continuously producing in the absence of said waves local oscillations of fixed amplitude and of fixed frequency equal to the center frequency of said modulated Waves, super-audibly quenching said oscillations, collecting angular velocity-modulated carrier waves, modulating the frequency of said locally produced oscillations in accordance with the frequency of the said collected "waves, providing an electron stream, and modulating said electron stream with said frequency modulated locally produced oscillations thereby vto provide an electron current 1 whose intensity varies in accordance with the modulation, signal of said collected waves,

ll. In combination with an electron discharge tube including a cathode, an output electrode and a plurality of intermediate control electrodes arranged in successive order in the electron stream between the cathode and output',electrode, an oscillatorynetwork connected between the cathode and two of th intermediate electrodes to provide a local oscillator normallyfproducing oscillations of constant amplitude and of a frequency equal to substantially the center frequency of applied angular velocity-modulated carrier waves, means for -applying such waves to said oscillator network, a time constant network'connected in the oscillatory network to provide super-regenerative detection of said applied waves, and an output circuit connectedto the output electrode consisting of a modulation signal voltage utilization means.

12. In combination with an electron discharge tube including a cathode, an output electrode anda pluralityof intermediate control grids arranged in successive order inthe electron stream between the cathode and output electrode, an

oscillatory network connected between the cathode and two of the intermediat grids to provide a local oscillator resonant to substantially the center frequency of appliedangular velocitymodulated carrierrwaves, means for applying such waves directly upon said oscillator network, means for tuning the oscillatory network over a wide frequency range, and a modulating signal voltage output circuit connectedto said output electrode. Y

13; A method of detecting frequency modulated carrier waves. which includes continuously proat a super-audible rate, providing an electron stream, and modulating said electron stream with said frequency modulated locally produced oscillations thereby to provide an electron current Whose intensity varies in accordance with the modulation signal of said collected waves.

SEYMOUR HUNT. 

